Guide for flashing your Android to stock here: http://forums.androidcentral.com/nexus-7-rooting-roms-hacks/191477-guide-nexus-7-factory-image-restore.html
Note this will wipe your Nexus clean, save anything you plan to keep first. To revert to 4.1 you need to download the 4.1.2 (JZO54K) factory...